1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Discussion of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a lithographic apparatus there is inevitably some absorption of the radiation as it passes through the projection system, such as a projection lens, which images the patterned radiation onto the substrate. This causes heating of the projection lens which results in lens aberration and so is detrimental to the imaging performance. Some attempts have been made to alleviate this, for example, by using feed-forward models which endeavour to predict the heating effect and resultant aberration such that compensation can be applied to correct for the predicted aberration. However, current models are only approximate and coarse, for example only using the average reticle transmission, illumination mode and numerical aperture as parameters. This excludes diffraction effects of the reticle and may also not be a valid model for more sophisticated reticles such as ones with differential attenuation and phase-shift masks. This leads to large residual errors after the feed-forward model is applied. Consequently, feed-back measurements of the actual aberration are periodically necessary in order to correct for the residual errors in the predicted lens aberration, such that the performance of the projection lens can be maintained within the required specification. However, these feed-back measurements result in a significant throughput penalty.
In order to predict more accurately the effect of lens heating, caused by the passage of radiation, on the aberration of the projection lens, one needs information on the spatial distribution of radiation within the projection lens. Radiation enters the projection lens at a variety of angles and this determines the spatial distribution of radiation at a pupil plane within the projection lens. The two dimensional intensity distribution at the pupil plane is known as the pupil filling.
Another problem is that some chips can be very complex, for example with a DRAM part and a logic part. The corresponding reticle will have different transmission and different features for the different parts, which leads to different pupil filling along the slit used in a scanner apparatus. Similarly, if only a small chip is being imaged, only a portion of the field will be bright and the rest will be dark. There is a problem regarding how to include these effects in the feed-forward model; if they are not included, this can cause errors in the calculated aberration.
A further factor is that measurement of the pupil filling of the projection lens has previously required extra items to be inserted into the machine and/or extensive measurement analysis. This can mean that the measurement is slow or indirect. Measurement of the scanning pupil in a scanner has also been difficult.